Search for the decays $\eta \to \mu e$ and $\eta \to e^{+}{e}^{-}$

A search for the lepton-family-violating decay $\eta \to \mu e$ and the rare decay $\eta \to e^{+}{e}^{-}$ yields the following branching ratio ($B$) upper limits at a 90% confidence level: $B(\eta \to \mu e)<\mathrm{}6\mathrm{}\times {10}^{-6\mathrm{}}$ and $B(\eta \to e^{+}{e}^{-})<\mathrm{}2\mathrm{}\times {10}^{-4\mathrm{}}$. This is the first direct search for $\eta \to \mu e$. The measurements were carried out at the SPES2 tagged $\eta$ facility at Laboratoire National Saturne in the course of a measurement of $B(\eta \to {\mu }^{+}{\mu }^{-})$.